1. Field of the Invention
This invention relates generally to electrophotography and more particularly, to a method for electrophotography wherein after an electrostatic latent image formed on a surface of a photoconductive and photosensitive member has been developed with toner, a toner image is transferred onto a recording medium.
2. Description of the Prior Art
According to a known method of electrophotography, a surface of a photoconductive and photosensitive member is uniformly charged, the uniformly charged surface of the photosensitive member is exposed to an imaging light to form a latent image of electric charge on the surface, and the latent image of electric charge is developed with toner. In such an electrophotography, for repetitious use of the photoconductive and photosensitive member, it is general to transfer the toner image formed on the surface of the photoconductive and photosensitive member onto the recording medium (recording paper). In some applications such as reproducing a positive image from a negative image, recording character information displayed on a cathode ray tube (CRT) or laser beam printing, use is made of reversal characteristics wherein the surface of the photoconductive and photosensitive member is deposited with toner at regions where electric charge are struck off by being exposed to light.
The electrophotography utilizing reversal and transfer techniques is carried out as follows. A photoconductive and photosensitive member is charged from a corona charger to uniformly provide positive charge over its surface (uniform charging process). The photoconductive and photosensitive member thus charged uniformly over its surface is then exposed to an imaging light through an optical system and loses its charge to form a latent image of electric charge (imaging light exposure process). The photoconductive and photosensitive member formed with the latent image of electric charge is then subjected to reversal (developing process). A device for reversal has a magnetic roller whose surface is applied with a magnetic brush constituted by a developer which contains a magnetic carrier and a toner, the toner being charged with electric charge of the same polarity (positive) as the electric charge forming the latent image. When the magnetic brush softly rubs against the surface of the photoconductive and photosensitive member, the toner is deposited on the regions of the photosensitive member surface where electric charge has been struck off (reversal process), thereby forming a toner image. A recording medium is then placed on the surface of the photoconductive and photosensitive member and a transfer field is applied on the back of the recording medium from a corona charger. The transfer field applies negative charge on the back since the toner image is of positive charge. Finally, the recording medium is separated from the surface of the photoconductive and photosensitive member with the toner image transferred onto the surface of the recording medium (transfer process).
Where it is desired to form again an image on the same photoconductive and photosensitive member, there is incorporated an additional process for erasing the latent image charge and the remaining toner image as well.
In general, repetitions and prolonged use of the photoconductive and photosensitive member is responsible for tendency toward degraded characteristic (wear) of the photosensitive member. This wear found expression in a problem which faces not only an increased residual potential wherein the surface potential will not decrease below a certain level even with an increased exposure but also an aggravated dark decay wherein the sustaining rate for the surface potential is decreased even in the dark owing to an increased dark current.